Fluent Nhibernate HasMany Duplicate Insert Problem

Question

I Have Duplicate Insert Problem. When i try insert duplicate rows

public class CostRegister : Entity<int>
{
    public virtual IList<CostDocumentRegister.CostDocumentRegister> CostDocumentRegister { get; set; }
}

public class CostDocumentRegister : Entity<int>
{
    public virtual string TaxNo { get; set; }
    public virtual string VendorCode { get; set; }
    public virtual CostRegister.CostRegister CostRegister { get; set; }
}

public CostDocumentRegisterMap()
{
    Table("[CostDocumentRegister]");
    Id(x => x.ID, "CostDocumentRegisterID").GeneratedBy.Native();
    Map(x => x.TaxNo, "TaxNo").CustomSqlType("nvarchar(20)").Not.Nullable();
    Map(x => x.VendorCode, "VendorCode").CustomSqlType("nvarchar(20)").Not.Nullable();

    References<CostRegister>(x => x.CostRegister, "CostRegisterID").Cascade.SaveUpdate().Not.LazyLoad().Not.Nullable();
}

public CostRegisterMap()
{
    Table("[CostRegister]");
    Id(x => x.ID, "CostRegisterID").GeneratedBy.Native();

    HasMany(x => x.CostDocumentRegister).Table("CostDocumentRegister").KeyColumn("CostRegisterID").Cascade.SaveUpdate().Not.LazyLoad();
}

The child table adds to the parent table as much as the number of elements.